The quantum computer is a computer that has an overwhelmingly rapid computing speed in solving particular problems that could not be solved in reality by conventional classical computers. In the quantum computer, a quantum two-level system called the quantum bit or qubit is utilized to correspond to a bit in a classical computer. While a number of qubits are used in computation, the most basic operation is carried out by unitary transformation manipulations for any one qubit and with a quantum operating device that reads out the qubit manipulated. In a solid-state electronic device, physical states proposed as usable for such qubits are superconducting, electronic and nuclear spin states.
At the outset, an explanation is given of basic particulars of qubits.
In general, if there are two physical states corresponding, respectively, to |0> and |1>, a state of superposition given by their superposition |0>+|1> functions as a qubit. Thus, while a classical bit is either 0 or 1, qubits other than |0> or |1> state include innumerable states intermediate between |0> and |1> and further those which are different in phase. It is called unitary transformation to let such a certain state |s> change to another state |s′>.
Qubits constituting a quantum computer need to equip the following four functions:
The first is initialization, requiring a means to set an initial state of a qubit as a well defined one, e.g., |0> or |1>.
The second is controlling a state (quantum operating gate), requiring a means to unitarily transform a prepared initial state (e.g., |0> or |1>) to any state of superimposition as desired |s>.
The third is to read out, requiring a detecting means to measure a unitarily transformed state |s>, namely to determine the values of amplitudes of |0> and |1>.
The fourth relates to expandability, requiring the conditional state control (controlled NOT gate) first on two bits and then requiring expansion by integration further to a number of qubits.
As quantum operating devices using superconducting qubits, there is a proposal to utilize electron pair boxes as two superconducting states having different charge states. There is also a proposal to utilize a superconducting quantum interference device (SQUID) to measure superconducting states having states different in phase.
In non-patent references 1 to 3 listed below, a theoretical proposal of a qubit consisting of a superconducting ring with three Josephson junctions and the detection of bonding and antibonding states in the proposed qubit have been reported. In this qubit, if an external magnetic field corresponding to half a unit magnetic flux is applied to the superconducting ring, two states degenerate in energy are realized. As a result, a bonding or an antibonding state that is any arbitrary state of superposition as desired of the second function mentioned above for qubits is formed. In such degenerate states, currents mutually opposite in direction flow through the superconducting ring. Thus, the superconducting ring to which an external magnetic field near the magnetic field corresponding to one half the unit magnetic flux is applied is irradiated with a microwave corresponding to an energy difference between the bonding and antibonding states, and a superconducting quantum interference device disposed around the quantum bit constituted of the superconducting ring is used to indirectly measure current flowing through the superconducting ring, thereby detecting if the state is bonding or antibonding.
In non-patent reference 4 in the list below, a theoretical proposal has been made on a qubit using a Josephson junction formed of an anisotropic (d-wave) superconductor and an isotropic (s-wave) superconductor. In this Josephson junction, by the effect of the anisotropic (d-wave) superconductor, its free energy becomes the minimum and its system becomes stable if the phase difference of the superconducting gap is ±π/2. The proposed qubit is used to arbitrarily superpose the bonding and antibonding states formed of these two degenerate states as the second function mentioned above for qubits.
In non-patent reference 5 in the list below, there have been reported a theoretical proposal on a qubit constituted by a superconducting ring with one ferromagnetic π-junction and four 0-junctions and reference to the qubit using an anisotropic superconductor discussed in non-patent reference 3. It is shown that the free energy of this system has its minimum when the phase difference of the superconducting gap is ±π/2, since the π junction large in the proportion of Josephson function is disposed between the two pairs of 0-junctions. The proposed qubit is used to arbitrarily superpose the bonding and antibonding states formed of these two degenerate states as the second function mentioned above for qubits.    Nonpatent Reference 1: J. E. Mooij and five others, “Josephson Persistent-Current Qubit”, SCIENCE, vol. 285, pp. 1036 (1999);    Nonpatent Reference 2: Caspar H. van der Wal and seven others, “Quantum Superposition of Macroscopic Persistent-Current States”, SCIENCE, vol. 290, pp. 773 (2000);    Nonpatent Reference 3: I. Chiorescu and three others, “Coherent Quantum Dynamics of a Superconducting Flux Qubit”, SCIENCE, vol. 299, pp. 1869 (2003);    Nonpatent Reference 4: Lev B. Ioffe and four others, “Environmentally decoupled sds-wave Josephson junction for quantum computing”, Nature, vol. 398, pp. 679 (1999); and    Nonpatent Reference 5: G. Blatter and two others, “Design aspects of superconducting-phase quantum bits”, Physical Review B, vol. 63, pp. 174511-1 (2001).